Another approach could be to create a scenario where the Assassins' Creed's hidden blade technology is discovered in the Black Lagoon world. Different factions in Black Lagoon would then fight over this new and dangerous weapon. A character like Rock could get caught in the middle, trying to prevent all - out war, while an Assassin tries to retrieve the technology to keep it from falling into the wrong hands.
A fanfic could also focus on a character from Black Lagoon being trained by an Assassin. For example, Dutch could be trained in the ways of the Assassins. He could use these new skills to expand his influence in the underworld, but also face moral dilemmas as the Assassin's Creed has its own code of ethics which might conflict with his more ruthless ways in the Black Lagoon world.
One idea could be to have a character from the Assassin's Creed world end up in the Naruto universe. For example, an Assassin could be sent on a mission that somehow transports them to Konoha. They would have to adapt to the new world with its unique chakra system while still relying on their assassin skills like stealth and parkour. This could lead to interesting interactions with Naruto and his friends as they try to figure out where this strange newcomer is from.
Don't forget about the dialogue. In Assassin's Creed, the language can be more formal and historical in some cases, while Black Lagoon has a lot of rough and colorful language. Blend these two styles in a believable way when your characters interact. Also, build a good plot structure with a clear beginning, middle, and end. Maybe start with a small event that leads to a bigger conspiracy involving both universes, and resolve it in a satisfying way at the end.
One idea could be that Naruto is somehow transported to the world of Assassin's Creed Odyssey. He could use his ninja skills, like his shadow clones, in the combat system of Odyssey. Maybe he teams up with the assassins to fight against a common enemy, such as a powerful cult that has some similarities to the Akatsuki in Naruto.
One idea could be that Naruto, with his unique ninja skills, is somehow transported to the world of Assassin's Creed Syndicate. He might mistake the Assassins for a new type of ninja clan. He could then learn their ways of stealth and parkour, while also teaching them some of his jutsu. For example, his shadow clone jutsu could be used to create distractions during assassinations.
A fanfiction could involve a character from Watch Dogs being drawn into the world of Assassin's Creed. Maybe they accidentally stumble upon an ancient Assassin's hideout while exploring an abandoned part of the city. There, they find artifacts and secrets that lead them on a journey to learn about the Assassins' ways and get involved in their age - old struggle against the Templars.
Another possibility is a time travel where the character goes to the era of Ezio Auditore but as an outsider. They might have to adapt to the Renaissance setting quickly while also trying to figure out how their presence could potentially change the events that Ezio is a part of. Maybe they are there to prevent a Templar plot that was never fully explored in the original games.
Another angle for a fanfic about Xenia could be her relationship with the protagonist. Perhaps they form an unlikely alliance based on mutual respect and a common goal. They could learn from each other's skills and experiences, and this could lead to a strong bond between them as they journey through the game's world together.
It could be really exciting. Imagine the stealth and parkour skills from Assassin's Creed in the complex and dangerous world of Game of Thrones. For example, an assassin could use their skills to navigate the alleys of King's Landing or scale the walls of Winterfell. They might be involved in the power struggles, perhaps trying to assassinate a key figure to change the course of the political intrigue, just like in the games and the show.
One idea could be a story where Kassandra, during one of her adventures, stumbles upon a mysterious portal that leads her to the Underworld. There, she meets Hades. Hades is initially suspicious of her but soon realizes she has no ill intent. They could form an unlikely alliance to deal with a threat that endangers both the mortal world and the Underworld.
One idea could be to explore the backstory of a minor character in the game. For example, a nameless assassin who has a unique skill but was never fully developed in the official story. Another could be to create an alternate timeline where the Templars and Assassins form an alliance against a greater threat.